Unified S band

Basis

The previous programs, Mercury and Gemini, had separate radio systems for voice, telemetry, and tracking. Uplink voice and command, and downlink voice and telemetry data were sent via ultra high frequency (UHF) and very high frequency (VHF) systems. The tracking capability was a C band beacon interrogated by a ground-based radar. With the much greater distance of Apollo, passive ranging was not feasible, so a new active ranging system was required. Apollo also planned to use television transmissions, which were not supported by the existing systems. Finally, the use of three different frequencies complicated the spacecraft systems and ground support. The Unified S-band (USB) system was developed to address these concerns.

The USB system did not completely replace all other radio transmitters on Apollo. While it was the sole mode for deep space communications, Apollo still used VHF for short range voice and low rate telemetry between astronauts and the Lunar Module (LM) and lunar rover during extra-vehicular activity; between the LM and Command/Service Module (CSM or CM); and between the CSM and Earth stations during the orbital and recovery phases of the mission. The CM had a backup capability to range the LM over its VHF voice links.

Apollo also carried several radars that operated independently of the USB on their own frequencies, including the landing and rendezvous radars on the LM and a C-band radar transponder on the CM.

Technical summary

From a NASA technical summary:

The design of the USB system is based on a coherent doppler and the pseudo-random range system which has been developed by JPL. The S-band system utilizes the same techniques as the existing systems, with the major changes being the inclusion of the voice and data channels.

A single carrier frequency is utilized in each direction for the transmission of all tracking and communications data between the spacecraft and ground. The voice and update data are modulated onto subcarriers and then combined with the ranging data [...]. This composite information is used to phase-modulate the transmitted carrier frequency. The received and transmitted carrier frequencies are coherently related. This allows measurements of the carrier doppler frequency by the ground station for determination of the radial velocity of the spacecraft.

In the transponder the subcarriers are extracted from the RF carrier and detected to produce the voice and command information. The binary ranging signals, modulated directly onto the carrier, are detected by the wide-band phase detector and translated to a video signal.

The voice and telemetry data to be transmitted from the spacecraft are modulated onto subcarriers, combined with the video ranging signals, and used to phase-modulate the downlink carrier frequency. The transponder transmitter can also be frequency modulated for the transmission of television information or recorded data instead of ranging signals.

The basic USB system has the ability to provide tracking and communications data for two spacecraft simultaneously, provided they are within the beamwidth of the single antenna. The primary mode of tracking and communications is through the use of the PM mode of operation. Two sets of frequencies separated by approximately 5 megacycles are used for this purpose [...]. In addition to the primary mode of communications, the USB system has the capability of receiving data on two other frequencies. These are used primarily for the transmission of FM data from the spacecraft.

Frequencies

The Unified S-Band System used the 2025-2120 MHz band for uplinks (earth to space transmissions) and the 2200-2290 MHz band for downlinks (space to earth transmissions). Both bands are allocated internationally for space research and operations, though by 2014 standards the ALSEP uplink was in the wrong part of the band (Deep Space instead of Near Earth).

Each Apollo spacecraft was assigned a frequency pair. For certain phase modulation (PM) downlinks, the uplink to downlink frequency ratio was exactly 221/240. The ALSEP lunar surface experiments shared a common uplink and did not, insofar as is known, implement a coherent transponder. (The passive laser retroreflectors left by the Apollo 11, 14 and 15 missions provide much greater accuracy, and have far outlived the active electronics in the other ALSEP experiments.) The Lunar Communications Relay Unit (LCRU) on the Lunar Rover had its own downlink frequency (to avoid interference with the LM) but shared the LM's uplink frequency as it did not implement a coherent transponder. To keep the VHF transmitters on the LM and LCRU from both trying to relay uplink voice and interfering with each other, separate voice subcarriers were used on the common S-band uplink: 30 kHz for the LM and 124 kHz for the LCRU.

The CSM had two separate transmitters, one PM and one FM. The LM had only one S-band transmitter that could operate in PM or FM, but not both simultaneously.

The S-IVB upper stage had its own USB transponder so it could be tracked independently after Apollo spacecraft separation until the stage either flew past the moon (Apollos 8, 10, 11, 12) or, starting with Apollo 13, hit the moon. This tracking data greatly aided the analysis of the impact as recorded by the seismometers left by earlier Apollo crews.

The S-IVB shared its S-band frequency pair with the LM. This created no problem in a normal mission as the LM remained dormant until lunar orbit, by which time the S-IVB had already hit the moon or flown off into orbit around the sun. However, it created an operational problem during the Apollo 13 mission when the LM had to be used as a lifeboat well before Apollo and the S-IVB reached the moon.

The LM frequency pair was also used by the subsatellites left in lunar orbit by the later J-missions. They were deployed by the CSM shortly before leaving lunar orbit and after the LM had completed its mission.

The use of two separated frequency bands made full duplex operation possible. The ground and the spacecraft transmitted continuously. Microphone audio was keyed either manually or by VOX, but unlike ordinary half duplex two-way radio both sides could talk at the same time without mutual interference.

Modulation

The S-band uplinks and downlinks usually (but not always) used phase modulation (PM). PM, like FM, has a constant amplitude (envelope) regardless of modulation. This permits the use of nonlinear RF amplifiers that can be considerably more efficient than RF amplifiers that must maintain linearity.

The PM modulation index is small, on the order of 1 radian, so the modulated signal more closely resembled double sideband amplitude modulation (AM) except for the carrier phase. A PM signal can be approximated for analysis purposes as an AM signal with the carrier (and only the carrier) rotated 90 degrees from its original phase. One important difference is that in AM, the carrier component has a constant amplitude as the sidebands vary with modulation while in PM the total signal (the envelope) is constant amplitude. This means that PM shifts power from the carrier to the information-carrying sidebands with modulation, and at some modulation indices the carrier can disappear completely. This is why Apollo uses a low modulation index: to leave a strong carrier that can be used for highly accurate velocity tracking by measurement of its Doppler shift.

Coherent transponders and Doppler tracking

Allocating uplink/downlink frequency pairs in a fixed ratio of 221/240 permitted the use of coherent transponders on the spacecraft. The spacecraft tracked the uplink carrier with a phase locked loop and, with a series of frequency dividers and multipliers, multiplied the uplink carrier frequency by the ratio 240/221 to produce its own downlink carrier signal.

When no uplink was detected, the transponder downlink carrier was generated from a local oscillator at the nominal frequency.

This "two-way" technique allowed extremely precise relative velocity measurements (in centimeters/sec) by observing the Doppler shift of the downlink carrier without a high accuracy oscillator on the spacecraft, although one was still needed on the ground.

Subcarriers

As mentioned above, the uplink and downlink carriers played a critical role in spacecraft tracking. Sidebands generated by the information also carried by the system had to be kept away from the carriers to avoid perturbing the phase locked loops used to track them. This was done through the use of various subcarriers.

The uplink had subcarriers at 30 kHz and 70 kHz. The 30 kHz subcarrier was modulated with uplink (Capcom) voice using narrowband FM (NBFM) and the 70 kHz carrier was modulated with command data for the onboard computer. This latter capability, which could be blocked by the astronauts, was used primarily to update the state vectors maintained by the computers with accurate values determined by ground tracking. It was also used to execute maneuvers in an unmanned spacecraft, e.g., deorbiting the lunar module after it had been jettisoned in lunar orbit.

Either or both subcarriers could be turned off when not needed, e.g., the voice subcarrier could be turned off during astronaut sleep periods. This improved the signal margins for the other information streams such as telemetry data.

The downlink normally had subcarriers at 1.25 MHz (NBFM voice) and 1.024 MHz (telemetry data). The telemetry could be at one of two rates, 1.6 kilobits/sec (low rate, 1/640 of the subcarrier frequency) and 51.2 kilobits/sec (high rate, 1/20 of the subcarrier frequency). High rate was used unless low rate was forced by poor link conditions, e.g., the use of a small earth receiving antenna, an omni spacecraft antenna, or the need to conserve spacecraft power by turning off its RF power amplifier. (The S-band transponder on the S-IVB had no voice subcarrier.)

A "backup voice" mode was available that shut off the 1.25 MHz NBFM voice subcarrier and transmitted voice at baseband on the main PM S-band carrier. This provided a few more dB of margin when the link was unusually degraded but worse voice quality than the normal voice mode when conditions were good.

The two modes can be easily distinguished by how they react to signal fades. In the normal (NBFM subcarrier) voice mode the audio SNR is usually very high. But as the link degrades below threshold, impulse or "popcorn" noise appears suddenly and builds up rapidly until it overwhelmed the astronauts' voices. An excellent example occurred during the Apollo 11 lunar landing when the lunar module structure occasionally obstructed the antenna's view of Earth.

The backup voice mode behaved more like AM; there is a constant background "hiss" and the astronauts' voices vary with signal strength. This mode was used extensively during the Apollo 13 emergency to conserve battery power in the LM Aquarius and during Apollo 16 because of the failure of the steerable S-band antenna on the lunar module Orion.

Voice transmissions used Quindar tones for in-band signaling.

Emergency key

The Apollo USB downlink also provided an "emergency key" capability consisting of a manually on-off keyed subcarrier oscillator at 512 kHz. Presumably this would have been used by the crew to transmit Morse Code if the downlink were too severely degraded to support even the backup voice mode. Although this mode had been tested (on Apollo 7) and most of the astronauts were trained in its use, this mode was never actually needed during any Apollo mission. (Apollo 13 made extensive use of the "backup voice" mode, as did the Apollo 16 lunar module Orion due to a failed high gain antenna).

A similar uplink capability did not exist because the uplink budget had far more margin than the downlink. A typical Apollo S-band spacecraft transmitter produced 20 watts; a typical uplink transmitter produced 10 kW, a ratio of 27 dB.

Ranging

The Apollo S-band system provided for accurate range (distance) measurements. The ground station generated a pseudorandom noise (PN) sequence at 994 kilobit/s and added it to the baseband signal going to the PM transmitter. The transponder echoed this PN signal back to earth on the downlink, and by correlating the received and transmitted versions the precise round trip light time to the spacecraft could be determined very accurately (within 15 meters).

The PN sequence, although deterministic, had the properties of a random bit stream. Although the PN sequence was periodic, its period of about 5 seconds exceeded the largest possible round trip time to the moon so there would be no ambiguity in its received timing.

Modern GPS receivers work somewhat similarly in that they also correlate a received PN bit stream (at 1.023 Mbit/s) with a local reference to measure distance. But GPS is a receive-only system that uses relative timing measurements from a set of satellites to determine receiver position while the Apollo USB is a two-way system that can only determine the instantaneous distance and relative velocity. However, an orbit determination program can find the unique spacecraft state vector or orbital element set that most closely matches (usually in a least squares sense) a set of range, range-rate (relative velocity) and antenna look angle observations made over a period of time by one or more ground stations assuming purely ballistic spacecraft motion over the observation interval.

Once the state vector has been determined, the spacecraft's future trajectory can be fully predicted until the next propulsive event.

Transponder ranging turn-around had to be manually enabled by an astronaut. It consumed an appreciable fraction of the downlink capacity and it was only needed occasionally, typically during handover from one ground station to the next. After the new uplink station achieved a 2-way coherent transponder lock in which the spacecraft generated its carrier at 240/221 times the received uplink frequency, the new ground station ranged the spacecraft. Then the ranging signal was turned off and the range measurement was continually updated by Doppler velocity measurements.

If for some reason a ground station lost lock during a pass, it would repeat the ranging measurement after re-acquiring lock.

FM and video

The normal operating mode of an Apollo S-band downlink transmitter was PM. This mode provided for coherent Doppler tracking, uplink commands, downlink telemetry and two-way voice—but not television. Video signals, even that from the slow scan camera used during the Apollo 11 EVA, are much wider in bandwidth than the other Apollo downlink signals. The PM link margin simply could not provide an acceptable picture, even when the largest available dishes were used.

A means was also needed to transmit wideband engineering and scientific data, such as that recorded on a tape recorder and played back at high speed.

The answer to both needs was wideband FM - frequency modulation. FM with a large modulation index exhibits a capture or threshold effect. The output signal-to-noise ratio (SNR) can be significantly greater than the RF channel SNR provided that the RF SNR remains above a threshold, typically around 8-10 dB.

This enhancement comes at a price: below the FM threshold, the output SNR is worse than the RF channel SNR. Reception is "all or nothing"; a receiving antenna too small to capture the video cannot capture the narrowband elements either (e.g., voice).

The CSM carried separate FM and PM transmitters that could operate simultaneously, so voice and telemetry continued to be transmitted by PM while the video came down by FM. The LM only carried a single transmitter that could operate in either FM or PM, but not both. FM cannot be used for Doppler tracking, so the LM always transmitted PM during flight, reserving FM for when video was required (e.g., during a surface EVA).

Until the transition to digital, satellite television also used wideband FM.

Interception

It is historically understood that the USSR did intercept the Apollo missions telemetry on the territory of the USSR, but until 2005, no source in the former USSR military or intelligence services has come forth with any evidence of this happening. The USSR used different frequency bands for its own space missions, so by default its deep space network did not readily have equipment able to receive Apollo telemetry. Whether China or any other non-Western (or non-aligned) country at the time chose to intercept any of the Apollo telemetry is unclear. Amateur radio and affiliated telecommunications sector persons could listen to the Apollo telemetry the world over—provided they could afford the reception equipment.

Within the territory of the US it was legally possible for amateur radio operators to monitor the telemetry, but the FCC did issue a directive that required all disclosure of Apollo telemetry interception be cleared by NASA. Paul Wilson and Richard T. Knadle, Jr. received voice transmissions from the Command Service Module of Apollo 15 in lunar orbit on the morning of August 1, 1971. In an article for QST magazine they provide a detailed description of their work, with photographs. At least two different radio amateurs, W4HHK and K2RIW, reported reception of Apollo 16 signals with home-built equipment.

Design influences

The International Space Station, Skylab as well as other orbital space stations have (or have had) some kind of unified microwave communications subsystem. The lasting engineering influence of the USB is that almost every human mission in space has had a unified microwave communications system of some kind.